The present invention relates to a golf ball with a core and a cover surrounding the core, particularly to a golf ball superior in spinning performance and impact resilience. In more detail, the present invention relates to a golf ball having high impact resilience and one which is relatively resistant to exhibiting a flier shot even when played from the rough. The present invention also relates to a method of manufacturing such a golf ball.
Conventionally, solid golf balls having a core with polybutadiene rubber as the base material surrounded by a cover resin and thread-wound golf balls having rubber thread wound around a core and enveloped by a cover resin such as ionomer are widely used in tournament matches.
During play, the golf ball often sinks deeply into the grass or rough of the fairway. A shot from the rough will often produce a flier type shot, i.e., the phenomenon whereby the ball is lifted very high due to a decrease of the backspin caused by the presence of grass between the club and the ball when the ball is struck. This flier yields unsatisfactory control, particularly at the time of an approach shot.
This phenomenon is encountered in both solid golf balls and thread-wound golf balls. Particularly, the aforementioned golf ball which has the ionomer resin cover experiences the disadvantage that a spin cannot be easily applied to the golf ball, leading to a degradation in controllability, although it still maintains good impact resilience due to the high elastic modulus.
It is said that a golf ball with a balata cover, i.e., a trans 1,4-polyisoprene cover is relatively resistant to a flier shot and superior in spin control. This is due to its appropriate hardness and high spinning performance due to the crystalline polymer portion and the crosslinking rubber portion of the golf ball.
In the case where crosslinking is effected at a temperature higher than the melting point of trans 1,4-polyisoprene (50xc2x0 C.-60xc2x0 C.), the crystalline portion will not be formed, even if cooling is effected after crosslinking. As a result, appropriate hardness cannot be obtained.
Therefore, it was conventionally necessary to carry out vulcanization over a long period of time under special ambience.
Japanese Patent Laying-Open No. 54-99177, for example, discloses the art of effecting a crosslinking reaction by immersing a golf ball in a xanthate type super vulcanization accelerator solution. However, there are many problems in the manufacturing step since immersion in the solution renders the post-process difficult.
Also, Japanese Patent Laying-Open No. 6-54928 discloses a method of crosslinking balata with a metal salt of an unsaturated fatty acid and a crosslinking initiator. This method has the disadvantage that the core enveloped by the cover is degraded due to the heat generated during the reaction. Also, an appropriate hardness could not be obtained since the crystal growth is not effected. Furthermore, if an accelerator for vulcanization such as the generally-used sulfur is employed, a vulcanization temperature of at least 120xc2x0 C. is required. In this case, the desired hardness cannot be obtained since crystal growth is not achieved.
The present invention is directed to solving the above problems, and accordingly it is an object of the present invention to provide a golf ball improved in spinning performance and impact resilience, and a method of manufacturing such a golf ball.
According to one aspect of the present invention, a golf ball is provided having a core enveloped by a cover with trans 1,4-polyisoprene as the main component, including a super accelerator formed of dithiocarbamate and/or xanthate. The golf ball is vulcanized at a temperature lower than the melting point of trans 1,4-polyisoprene.
According to another aspect of the present invention, a golf ball is provided having a core enveloped by an inner layer cover and an outer layer cover. The inner layer cover, which envelopes the core, has trans 1,4-polyisoprene as the main component, and includes a super accelerator formed of dithiocarbamate and/or xanthate. The outer layer cover, which envelopes the inner layer cover, has trans 1,4-polyisoprene as the main component and includes a super accelerator of an amount larger than the amount in the inner layer cover.
According to a further aspect of the present invention, a golf ball is provided having a super accelerator formed of dithiocarbamate and/or xanthate.
According to still another aspect of the present invention, a golf ball is provided including an inner layer cover and an outer layer cover vulcanized at a temperature lower than the melting point of trans 1,4-polyisoprene.
According to a still further aspect of the present invention, a golf ball is provided including an outer layer cover and an inner layer cover having the ratio (W0/W1) of the thickness of the outer layer cover (W0) to the thickness of the inner layer cover (W1) in the range of 0.1-10.
According to yet a further aspect of the present invention, a golf ball is provided having a core which is a thread-wound core.
According to yet another aspect of the present invention, a method of manufacturing a golf ball including a core having a solid or thread-wound structure and a cover enveloping the core is provided, including the steps of blending a super accelerator for low temperature vulcanization in a polymer component with trans 1,4-polyisoprene as the main component to prepare a composition for the cover, molding the cover composition into a half shell, disposing a core enveloped by the half shell in a mold and forming dimple patterns at the surface of the cover composition, and vulcanizing the cover composition at a temperature lower than the melting point of trans 1,4-polyisoprene.
According to yet a still further aspect of the present invention, a method of manufacturing a golf ball is provided having a super accelerator formed of dithiocarbamate and/or xanthate.
According to still another aspect of the present invention, a method of manufacturing a golf ball is provided having a vulcanization temperature of 30xc2x0 C.-50xc2x0 C.
According to still another aspect of the present invention, a method of manufacturing a golf ball is provided including the steps of blending a super accelerator for low temperature vulcanization in a polymer component mainly composed of trans 1,4-polyisoprene to prepare a composition for an inner layer cover, blending a super accelerator of an amount larger than the amount in the inner layer cover in a polymer component mainly composed of trans 1,4-polyisoprene to prepare a composition for an outer layer cover, molding the inner layer cover composition into an inner half shell, molding the outer layer cover composition into an outer half shell, disposing a core enveloped by the outer layer half shell and the inner layer half shell in a mold, forming dimple patterns at the surface of the outer layer cover composition, and vulcanizing the inner layer cover composition and outer layer cover composition at a temperature lower than the melting point of trans 1,4-polyisoprene.
According to an additional aspect of the present invention, a method of manufacturing a golf ball is provided utilizing a super accelerator formed of dithiocarbamate and/or xanthate.
According to yet another aspect of the present invention, a method of manufacturing a golf ball is provided having a vulcanization temperature of 20xc2x0 C.-50xc2x0 C.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention.
In the present invention, the composition for the cover is mainly composed of trans 1,4-polyisoprene. Here, trans 1,4-polyisoprene refers to a component having at least 60 trans structures in the polymer molecule. If the number of trans structures is less than 60, the crystalline portion is so small that the melting point becomes too low to be able to satisfy the basic property as a cover. The other polymer used in the mixture of the cover material is preferably a resin, a rubber or a high impact resilient material having a partial crystalline structure such as a high styrene resin, high cis-polybutadiene, an ionomer resin, and the like. These polymers can be blended in the range of not more than 50% by weight, preferably not more than 25% by weight with respect to the polymer component of the cover material. If the amount of trans 1,4-polyisoprene contained therein is low, the spinning performance and controllability will be reduced.
The super accelerator used in the present invention is an accelerator that can have the reaction initiated at a temperature of 70xc2x0 C. or below, preferably at approximately 30xc2x0 C.
Dithiocarbamate and xanthate, for example, are known as super accelerators, and include the following compounds.
As dithiocarbamate, dimethyl*sodium dithiocarbamate, diethyl*sodium dithiocarbamate, di-n-butyl sodium dithiocarbamate, dimethyl*zinc dithiocarbamate, diethyl*zinc dithiocarbamate, di-n-butyl zinc carbamate, diamyl*zinc dithiocarbamate, dibenzyl*zinc dithiocarbamate, N-pentamethylene*zinc dithiocarbamate, methyl pentamethylene*pipecoline dithiocarbamate, pipecolin pipecolyldithiocarbamate, pentamethylene*dithiocarbamate and the like can be enumerated.
As xanthate, isopropyl*sodium xanthate, isopropyl zinc xanthate, butyl*zinc xanthate, and the like can be enumerated.
Examples of the ingredients of the cover composition employed in the present invention will be set forth in the following. In the case where resin and/or rubber other than trans 1,4-polyisoprene is employed, the composition can be formed of 5-20 parts by weight of natural rubber, 5-20 parts by weight of high styrene resin, 0-20 parts by weight of diene rubber, 1-10 parts by weight of zinc oxide, and 1-20 parts by weight of titanate oxide, with respect to 100 parts by weight of trans 1,4-polyisoprene. Additionally, fiber reinforced rubbers, fiber reinforced resins, inorganic single crystal components, weight adjusters, metal powder, metal oxide and the like can be appropriately mixed. An amount of 0.1-3 parts by weight of sulfur is also blended therein.
If the amount of sulfur is less than the above-specified range, vulcanization will not proceed sufficiently, so that sufficient strength for a golf ball cover cannot be obtained. If the amount of sulfur is less than the above-specified range, vulcanization will occur during mixture of the material by the roll or kneader. Therefore, the amount of sulfur is preferably 0.1-3 parts by weight, particularly 0.2-2 parts by weight.
The thickness of the cover is preferably in the range of 0.5-4.0 mm, particularly in the range of 0.8-3.0 mm. If the cover thickness is less than 0.5 mm, durability will be degraded. If the cover thickness exceeds 4.0 mm, the volume ratio of the cover component to the entire ball is increased to degrade that impact resilience of the golf ball.
In the case where the cover includes an inner layer cover and an outer layer cover, the entire thickness of the inner layer cover and outer layer cover is preferably in the range of 0.5-4.0 mm, particularly in the range of 0.8-3.0 mm. Also, the ratio (W0/W1) of the thickness of the outer layer cover (W0) to the thickness of the inner layer cover (W1) is in the range of 0.1-10, preferably in the range of 1-10. If W0/W1 exceeds 10, the anchor effect of the core by the inner layer cover cannot be expected. If W0/W1 is smaller than 0.1, vulcanization may be insufficient to degrade the basic property of the cover.
The super accelerator is blended 0.1-3.0 parts, preferably 0.2-2.0 parts, further preferably 0.5-2.0 parts, with respect to the polymer component of 100 parts of the cover composition. If the amount of the super accelerator is less than the above-specified range, vulcanization will not proceed sufficiently, so that sufficient strength for the golf ball cover cannot be obtained. When the amount of the super accelerator is greater than the above-specified range, vulcanization will occur during mixture of the material by the roll or kneader.
In the case where the cover includes an inner layer cover and an outer layer cover, the amount of the super accelerator (C1) blended in the polymer composition of the inner layer cover is less than the amount of the super accelerator (C2) in the polymer composition of the outer layer cover. The ratio thereof (C1/C2) is preferably in the range of 0.01-0.8, more preferably in the range of 0.1-0.6. If the ratio is lower than 0.01, the vulcanization reaction of the polymer composition of the inner layer cover will be insufficient. If the ratio is 1.0 or more, impregnation of the cover material in the core is so small that the anchor effect cannot be expected. The amount of the super accelerator blended in the polymer composition of the inner layer cover is 0.05-2.5 parts by weight, preferably 0.1-1.5 parts by weight with respect to 100 parts by weight of the polymer component. The amount of the super accelerator blended in the outer layer cover is 0.1-3.0 parts by weight, preferably 0.2-2.0 parts by weight.
A method of manufacturing a golf ball of the present invention is set forth in the following.
Mixing of the cover composition is carried out by a roll or kneader. Here, the appropriate temperature of the material is 50xc2x0 C.-80xc2x0 C. If the temperature is below 50xc2x0 C., trans 1,4-polyisoprene will not melt, so that other materials cannot be mixed therewith. If the temperature exceeds 80xc2x0 C., the reaction of the super accelerator will be initiated, causing crosslinking. The mixing time should be as short as possible. Specifically, 1-20 minutes is preferable depending upon the blended amount of the super accelerator. If the mixing time is shorter than the above-specified time, diffusion of the ingredient in the polymer composition is degraded. If the mixing time is longer than the above-specified time, an inappropriate crosslinking reaction will occur.
The foregoing cover composition can be used to envelope both a solid core or a thread-wound core. To provide the cover composition around the solid core or the thread-wound core, hemispherical half shells are produced in advance and press-formed over the core. In producing the half shell, it is desirable to carry out the process at a low temperature of 60xc2x0 C.-90xc2x0 C. so that the material can flow, followed by a rapid cooling process.
A dimple pattern is then formed at the surface of the ball with the core enveloped by the half shells. Here, it is preferable to carry out molding at the temperature of 60xc2x0 C.-90xc2x0 C., followed by a rapid cooling process.
At this current stage, vulcanization reaction has not yet occurred, or partial occurrence thereof is of a level that will not degrade the flow property of the material.
Then, the cover composition is vulcanized. This is carried out by the method of setting the press temperature to at least 30xc2x0 C. and not more than 50xc2x0 C., and completing the crosslinking reaction within the mold in 2-72 hours, or removing the cooled object from the mold after press-formation and completing the crosslinking reaction at a similar temperature and period of time in an oven. Here, the vulcanization temperature is preferably 30xc2x0 C.-50xc2x0 C. which is lower than the melting point of trans 1,4-polyisoprene. The time of vulcanization is approximately 2-72 hours, depending upon the amount and type of the ingredients of the super accelerator. The cover material of the resultant ball has the vulcanization reaction completely finished.
The above-described method is applicable to a multipiece core formed of one or more layers, or a core of a thread-wound structure having rubber thread wound around a solid center or a liquid center.
A solid core, for example, a solid core for a two piece ball, is produced as set forth in the following. With respect to 100 parts by weight of polybutadiene, a rubber composition including a total of 10-50 parts by weight of a crosslinking agent alone or of at least two types formed of xcex1, xcex2-monoethylenic unsaturated carboxylic acid such as acrylic acid or methacrylate or metal salt thereof and trimethylolpropane trimethacrylate polyfunctional monomer, 10-30 parts by weight of a filler such as zinc oxide or barium sulfate, 0.5-5 parts by weight of peroxide such as dicumyl peroxide, and, if necessary, 0.1-1 parts by weight of an antioxidant is heated and applied with pressure for 10-40 minutes at the temperature of 140xc2x0 C.-170xc2x0 C. using a press to form a spherical crosslink.
In contrast, a thread-wound core includes a center and a rubber thread layer formed by winding rubber thread in an elongated state around the center. The center may be a solid center composed of a vulcanized formation of a rubber composition, or a liquid center having water or liquid such as paste sealed in a center cover of vulcanized rubber. The solid center preferably has a diameter of 27-38 mm. If the diameter of the solid center is smaller than 27 mm, the shot angle becomes so small that the amount of spinning will increase. As a result, the flight distance cannot be increased. If the diameter of the solid center is greater than 38 mm, rubber thread cannot be wound sufficiently around due to limitation in the diameter of the golf ball. The thin rubber thread layer will degrade the impact resilience of the ball due to its insufficient repulsion. As a result, the flight distance cannot be increased. In the case where a liquid center is employed, the diameter is preferably 25-35 mm. If the diameter of the liquid center is smaller than 25 mm, the shot angle will become so small that the amount of spin increases. As a result, the flight distance cannot be increased. If the diameter of the liquid center is greater than 35 mm, the predetermined ball hardness cannot be readily obtained. Also, the rubber thread layer will become so thin that the impact resilience of the ball is degraded to suppress gain of a flight distance.
Rubber thread can be obtained by vulcanizing a rubber composition including a blend of an antioxidant, vulcanization accelerator, sulfur and the like with natural rubber or with blended rubber of natural rubber and synthetic polyisoprene.
The method of manufacturing a golf ball with a cover including an inner layer cover and an outer layer cover is set forth in the following.
Mixing the polymer composition of the inner layer cover and the outer layer cover is carried out as described above.
Then, the core is enveloped by the half shells of an inner layer cover and an outer layer cover that are molded in advance. Dimple patterns are formed at the surface of the ball in a mold. Here, it is preferable to carry out molding at the temperature of 60xc2x0 C.-90xc2x0 C., followed by rapid cooling. Formation of the half shell is desirably carried out at the low temperature of 60xc2x0 C.-90xc2x0 C., for example, where the polymer composition of the cover can flow, followed by rapid cooling.
At this current stage, the vulcanization reaction has not yet occurred, or partial occurrence thereof is of a level that does not degrade the flow property of the polymer composition.
Then, the polymer composition of the cover is vulcanized. The method is employed of setting the press temperature to at least 20xc2x0 C. and not more than 50xc2x0 C. and completing the vulcanization reaction in the mold in 5-80 hours, or removing the cooled object from the mold after press-formation to complete vulcanization at a similar temperature and period of time in an oven. Here, the vulcanization temperature is preferably 20xc2x0 C.-50xc2x0 C. which is a temperature lower than the melting point of trans 1,4-polyisoprene. The vulcanization period of time is approximately 5-80 hours, depending upon the amount and type of the ingredients of the super accelerator. The cover material of the output ball has the vulcanization reaction completely finished.
In addition to the above-described method of applying a molding process on a core enveloped by an inner layer cover and an outer layer cover, the method of enveloping the core by half shells of the inner layer cover to integrate the same in a mold to form an inner sphere, and then surrounding the same with the half shells of the outer layer cover for formation can be employed. Although the optimum conditions for molding can be selected for the inner layer cover and the outer layer cover, respectively, there is the undesirable step of removing the burr generated in the molding step of the inner layer cover, and then molding the outer layer cover.
The above-described method is applicable to the golf ball of both a solid core and thread-wound core.